title: Night-shift work and breast cancer--a systematic review and meta-analysis.
abstract: OBJECTIVE
  The aim of this review was to synthesize the evidence on the potential relationship between nightshift work and breast cancer.
  
  
  METHODS
  We searched multiple databases for studies comparing women in shift work to those with no-shift work reporting incidence of breast cancer. We calculated incremental risk ratios (RR) per five years of night-shift work and per 300 night shift increases in exposure and combined these in a random effects dose-response meta-analysis. We assessed study quality in ten domains of bias.
  
  
  RESULTS
  We identified 16 studies: 12 case-control and 4 cohort studies. There was a 9% risk increase per five years of night-shift work exposure in case-control studies [RR 1.09, 95% confidence interval (95% CI) 1.02-1.20; I (2) = 37%, 9 studies], but not in cohort studies (RR 1.01, 95% CI 0.97-1.05; I (2) = 53%, 3 studies). Heterogeneity was significant overall (I (2) = 55%, 12 studies). Results for 300 night shifts were similar (RR 1.04, 95% CI 1.00-1.10; I (2) = 58%, 8 studies). Sensitivity analysis using exposure transformations such as cubic splines, a fixed-effect model, or including only better quality studies did not change the results. None of the 16 studies had a low risk of bias, and 6 studies had a moderate risk.
  
  
  CONCLUSIONS
  Based on the low quality of exposure data and the difference in effect by study design, our findings indicate insufficient evidence for a link between night-shift work and breast cancer. Objective prospective exposure measurement is needed in future studies.
authors.affiliation: Cochrane Occupational Safety and Health Review Group, Finnish Institute of Occupational Health, Kuopio, Finland. sharea.ijaz@ttl.fi
authors.forename: Sharea
authors.initials: S
authors.lastname: Ijaz
authors.forename: Jos
authors.initials: J
authors.lastname: Verbeek
authors.forename: Andreas
authors.initials: A
authors.lastname: Seidler
authors.forename: Marja-Liisa
authors.initials: ML
authors.lastname: Lindbohm
authors.forename: Anneli
authors.initials: A
authors.lastname: Ojajärvi
authors.forename: Nicola
authors.initials: N
authors.lastname: Orsini
authors.forename: Giovanni
authors.initials: G
authors.lastname: Costa
authors.forename: Kaisa
authors.initials: K
authors.lastname: Neuvonen
mesh_terms: D000368:Aged; D001943:Breast Neoplasms / Q000378:metabolism* / Q000473:pathology; D016022:Case-Control Studies; D005260:Female; D006801:Humans; D008875:Middle Aged; D011960:Receptors, Estrogen / Q000378:metabolism*; D012307:Risk Factors; D014940:Work Schedule Tolerance
chemical_list: D011960:Receptors, Estrogen